Steam's midweek title the week is Plants Versus Zombies, a game which I got at 'full' price on Halloween weekend because I was convinced the price would never be halved (it's basically permanently half the price of the official version from PopCap themselves - when PopCap did their inevitable halloween promo, the difference was a £1 saving and no achievements).

£3.49 and an instant recommendation. Play the demo via Steam if you're not sure. It's slightly plodding in that first hour, but you'll get the idea if it's your sort of game, and it certainly gets more diverse and hectic as it goes on. Sunk about 30 hours into it myself.